Thin layer chromatography (TLC) is a technique known in chemistry to separate chemical species, and has a wide range of uses in the fields of pharmaceuticals, water quality analysis and organic synthesis. A stationary phase provides a tortuous flow path, and the thin layer is often formed from a material such as silica gel, alumina or cellulose, with the material selected for the porosity provided and its inert chemical nature, relative to the components of the liquid phase. A liquid phase consists of a mixture of compounds to be separated, dissolved in an appropriate solvent. The liquid phase is placed on the thin layer plate and is drawn up the plate by capillary action, with the mobility of the compounds in the mixture determined by the interactions of the analytes with the stationary phase.
Known impediments to wider use of the TLC technique include the cost and difficulty of fabricating the stationary phase, as well as the brittleness of it. A further impediment is the minimum size of the diameter and thickness available for a stationary phase plate fabricated from the conventional materials.
Solid phase microextraction (SPME) is an extraction technique that was developed as a fast method to analyze volatile and semivolatile compounds. SPME offers an inexpensive and solvent-free extraction method that is a viable alternative to liquid-liquid extractions and solid phase extractions. This technique was developed by Berladi and Pawliszyn in 1989.
A typical apparatus used in SPME comprises a silica rod or platinum wire that is coated with a specific extraction medium. SPME fibers often have a length of about 1 cm, and the desired thickness of the extraction medium ranges from a monolayer to about 100 μm. The coated fibers are used by placing the fiber in contact with the sample matrix, a bulk fluid or its surrounding headspace. The fibers can be used to analyze samples from the liquid phase, gaseous, or solid matrices. The samples are measured by direct exposure of the fiber to the bulk fluid or its surrounding headspace.
By developing a fiber coating with different functionalities, SPME fibers can be chosen for the specific analyte(s) of interest. This allows the performance of SPME to be optimized where only the analytes of interest are analyzed rather than having matrix effects interfering in the extraction. By decreasing matrix effects, the detection limit of the analytes can be lowered.
As described, the separation techniques of TLC and SPME have known impediments.